Touch screens are user interface devices employed with electronic visual displays that may detect the presence of a touch on or in the vicinity of a display area of a touch screen or panel. A known touch screen user interface employs transparent conductive electrodes composed indium tin oxide or ITO which overlay the display such as an LCD or other suitable display device.
The term touch screen generally refers to a transparent panel overlaying an display, such as an LCD, which may be touched with a finger, hand or stylus to produce an output signal or indication, and may also include so-called proximity or gesture detectors which can affect the output as well. When implemented to detect and identify a plurality of touches, such touch screen devices generally employ so called transmit electrodes Tx or Tx ouputs and receive electrodes Rx or Rx outputs. These electrodes are transparent and are laid out in an approximately orthogonal grid arrangement such that the Rx and Tx electrodes cross over each other resulting in inter-electrode or mutual capacitance. When a user touches the panel, the inter-electrode capacitance between the Rx and Tx electrodes changes. The Rx and Tx electrodes are periodically scanned by means of a touch screen controller having a processing unit (e.g., CPU) responsively coupled thereto. The changes in capacitance thus form sensible signals which may be interpreted by software in the CPU which performs selected functions.
The display located behind the transparent touch screen allows the user to view the images displayed by the display. The user thus has a convenient interface whereby they may touch the touch screen panel and the images viewed through the panel may be manipulated or interacted with, and changes may be viewed as successive images on the display. This results in a versatile interface which is currently in use in a wide variety of devices including touch screen computers, smart phones and personal digital assistants (PDAs). Unfortunately, the normal operation of the LCD often produces electrical noise which adversely affects or influences the capacitances measured by the touch screen controller. The noise produced by the display may be dependent on for example the displayed image, mechanical construction of the display, drive/refresh method of the display, materials used in the display and signaling dependent. This noise can interfere with or unduly influence the capacitances measured by the touch screen controller thereby resulting in errors in reported touches or locations of touches.
Various noise suppression approaches have been attempted, including synchronization using phase locked loop PLL systems. These tend to be expensive and difficult to implement, because they require complex circuitry and high processor overhead and speeds. Another solution has been to increase the Tx signal strength in the touch screen controller, but this decreases battery life. Other exemplary software and hardware approaches currently available have had mixed results, and each exhibit disadvantages in terms of cost, implementation and performance.
Another approach has been to physically shield the display noise from the touch screen by locating a transparent shielding electrode between the and the touch screen, or increasing the spacing between the display and the touch screen, or both. These approaches are direct and can be effective, but they often increase cost. More importantly these methods result in a thicker, more bulky touch screen device which may be commercially unattractive, because users seem to prefer slim, light weight devices.
Other techniques employ specialized firmware, which adds cost and reduces the competitiveness of the resulting system.